1. Field of the Invention
The present invention relates to a display device, and more particularly, to a liquid crystal display (LCD) panel and a method for manufacturing the same.
2. Discussion of the Related Art
Rapid development within the fields of information and communication has caused an increase in the demand for thin, lightweight and low cost display devices for viewing information. Industries that develop displays are responding to these needs by placing high emphasis on developing flat panel type displays.
Historically, the Cathode Ray Tube (CRT) has been widely used as a display device in applications such as televisions, computer monitors, and the like, because CRT screens can display various colors having high brightness. However, the CRT cannot adequately satisfy present demands for display applications that require reduced volume and weight, portability, and low power consumption while having a large screen size and high resolution. Out of this need, the display industry has placed high emphasis on developing flat panel displays to replace the CRT. Over the years, flat panel displays have found wide use in monitors for computers, spacecraft, and aircraft. Examples of flat panel display types currently used include the LCD, the electroluminescent display (ELD), the field emission display (FED), and the plasma display panel (PDP).
Characteristics required for an ideal flat panel display include a light weight, high luminance, high efficiency, high resolution, high speed response time, low driving voltage, low power consumption, low cost, and natural color.
Currently, the LCD device is widely used as a monitor for portable computers. These LCD's typically include two opposing glass substrates, between which is sealed a layer of liquid crystal. A plurality of pixel patterns is formed on one of the glass substrates, and a color filter layer is provided on the other substrate.
The two glass substrates are attached to each other using sealant that serves to attach and fix the two glass substrates to each other. Since liquid crystal has low specific resistance and easily absorbs moisture in the air, it is susceptible to incorporating impurities. Accordingly, the sealant used to attach the two glass substrates must also resist permeation of external moisture that can adversely affect the liquid crystal provided between the glass substrates.
Sealants presently used include inorganic and organic sealants. Inorganic sealant has been traditionally used for sealing an LCD. However, with currently developed liquid crystal materials, inorganic sealant generally is no longer used. Instead, organic epoxy based resins, such as phenol-based or acryl-based resins, are presently used as an LCD sealant. Epoxy based sealants can generally be divided into either a two-liquid state type that requires mixing a main material with a hardener, or a one-liquid state type that includes hardener already incorporated into the main material.
Depending on the hardening type, organic resin sealant is generally hardened by either a thermal process or exposure to ultra-violet (UV) rays. In either case, high adhesion strength, high crystallization ratio, and exceptional printing performance are required to obtain a seal with high reliability. Uniform distribution of organic sealant also is necessary to accurately control a liquid crystal cell gap while pressuring, heating, and hardening the glass substrates.
Thermal hardening resin has high mechanical strength, high adhesion strength, and high cross-linkage at a high temperature. Epoxy and phenol resins are mainly used as a thermal hardening resin. However, UV-type hardening resin is most often used for sealing large size display panels because, compared to thermal hardening resin, it hardens at low temperature, has reduced hardening time and improves adhesion. Moreover, when a thermal hardening resin is applied to large sized substrates, thermal expansion of the resins often occurs.
Various methods are currently used to inject liquid crystal between opposing glass substrates of an LCD. Two representative methods described below include a first method that injects liquid crystal into a cell using a pressure difference by maintaining a vacuum state within the cell, and a second method that distributes liquid crystal under a decompression state.
In the first method, a liquid crystal panel on which a sealant is printed is placed in a vacuum chamber. The chamber pressure is gradually reduced until an inner portion of the liquid crystal panel reaches a low-pressure state that is close to vacuum. While the inner portion of the liquid crystal panel is maintained at the low-pressure state, a liquid crystal injection hole is placed in contact with liquid crystal located outside the liquid crystal panel. Air is then introduced into the chamber to cause external pressure on the liquid crystal panel to gradually increase to a high level. Consequently, because of the existing pressure differential between the inner and outer portions of the panel, liquid crystal is injected into the panel under a vacuum state to form a liquid crystal layer between the first and second glass substrates.
In the second method for distributing liquid crystal, a sealant is first patterned on one of the glass substrates. After the sealant is patterned, liquid crystal is dropped onto the substrate using a dispenser. This method has an advantage in that the injection speed of the liquid crystal is higher compared to methods based on osmotic pressure. This method also has an advantage in that injection time of the liquid crystal is short for large sized substrates.
A related art LCD panel is now described with reference to FIGS. 1A to 1C. FIG. 1A is a plan view of a first substrate in which thin film transistors (TFTs) 17 and pixel electrodes 19 are patterned. FIG. 1B is a cross-sectional view I-I′ of the structure shown in FIG. 1A that shows a TFT 17, an extended gate line 13a, the connection of the pixel electrode 19 to a source/drain of the TFT 17, and the first substrate 11. FIG. 1C is a cross-sectional view of an second substrate 11a, on which are formed a color filter pattern 21, a light shielding layer 23 and a common electrode 25.
As shown in FIG. 1A, a first substrate 11 includes a plurality of gate lines 13a formed along a first direction on the substrate. A plurality of data lines 15a are formed along a second direction on the first substrate 11 and cross each of the gate lines 13a. TFTs 17 and pixel electrodes 19 are formed in an active region A at each crossing point between a gate line 13a and a data line 15a. Each of the gate lines 13a and the data lines 15a extend to a pad region located at the display periphery outside the active region and are respectively electrically connected with a gate driving circuit (not shown) and a data driving circuit (not shown).
FIG. 1B shows a cross-sectional view of a gate line extension from the active region A where the TFTs 17 and the pixel electrodes 19 are formed. As shown in FIG. 1C, the second substrate 11a includes the plurality of color filter patterns 21, the light-shielding layers 23 for shielding regions other than the pixel electrodes 19 from light, and the transparent common electrode 25.
FIGS. 2 and 3 are respectively simplified plan and cross-sectional views of the related art LCD after the first substrate 11 is joined with the second substrate 11a. As shown in FIG. 2, the light-shielding layer 23 is patterned into a matrix arrangement in the active region A. Light-shielding layer 23 also is formed along a periphery of the active region A and covers the sealing regions of the substrates where sealant 27 is formed.
Referring to FIG. 3, the first substrate 11 is shown provided with the gate lines 13a and the data lines 15a (not shown), and the second substrate 11a is shown provided with the light-shielding layers 23. The sealant 27 is formed outside the periphery of the active region A and attaches the first substrate 11 to the second substrate 11a. The sealant 27 is a thermal hardening type that forms a seal in a pad region P between the light-shielding layer 23 on the second substrate 11a and the gate lines 13a and data lines 15a (and areas therebetween) formed on the first substrate 11.
The related art LCD panel that utilizes thermal hardening type sealant has several drawbacks. First, a high temperature heat must be applied to the sealant in order for it to harden. Heating the substrates at high temperatures may cause an increase in substrate stress due to thermal expansion of the sealant. Another drawback is the long hardening time typically required for thermal hardening type sealant.
Furthermore, the current trend toward large size panels generally makes it more desirable to use UV hardening type sealant. However, since an upper portion of the sealing region S is covered with the light-shielding layer, the transmission of UV-rays are substantially prevented from directly irradiating the sealant, thus making it difficult or impractical to use UV hardening type sealant.
Thus, there remains a need in the art for an LCD panel sealing structures and processes that avoid the disadvantages associated with high temperature sealing processes or the manufacturing complexities currently associated with UV-type hardening sealant.